This invention relates to starting and operating method and apparatus for discharge lamps and, more particularly, to such starting and operating method and apparatus which operate discharge lamps on square waves at a relatively low frequency wherein the apparatus current-limiting components are included in circuit prior to the oscillator elements which form the operating square waves.
Miniature high-pressure metal-vapor discharge lamps are described in U.S. Pat. No. 4,161,672, dated July 17, 1979 To Cap et al. Such lamps are known to have potential utility as screw-in replacements for incandescent lamps. These lamps can also be operated with high efficacy at frequencies above the audible range. With such operation, there exist certain "windows" of permissible operating frequency as well as regions of instability for the arc and such a mode of high-frequency operation is disclosed in U.S. Pat. No. 4,170,746, dated Oct. 9, 1979 to Davenport. These lamps can also be operated from direct current. In the usual case, with DC operation or high-frequency operation, it is normally required that the available AC be rectified and filtered with a relatively large capacity aluminum electrolytic capacitor. Such filtering substantially reduces the power factor and aluminum electrolytic capacitors are sensitive to higher temperatures of operation and display a relatively short operating life, in some cases.
It is known to operated discharge lamps with a rectified current wherein the current-limiting impedance is placed into the line prior to rectification. Such a mode of operation is shown in U.S. Pat. No. 3,787,751, dated Jan. 22, 1974 to Farrow. A variety of such arrangements are also shown in U.S. Pat. No. 4,084,217, dated Apr. 11, 1978 to Brandli et al. and U.S. Pat. No. 4,187,449, dated Feb. 5, 1980 to Knoble.
When high-intensity-discharge lamps are first started, they normally require a few minutes to warm up and generate operating pressure within the envelope in order to produce full light output. In addition, after momentary periods of power interruption, the lamps must cool down before they can be reignited, after which the pressure must again build up to achieve rated light output. It is well known to provide standby incandescent lighting for such lamps and a wide variety of circuits are available. One such standby lighting system is disclosed in U.S. Pat. No. 3,517,254, dated June 23, 1970 to McNamara. Another type of such a standby lighting system is disclosed in U.S. Pat. No. 3,723,808, dated Mar. 27, 1973 to Jones. In U.S. Pat. No. 4,170,744, dated Oct. 9, 1979 to Hansler is disclosed a standby lighting system for use with a miniature metal-vapor lamp with the combination formed as a screw-in light source.
In the case of fluorescent lamps, it is known to operate such lamps with relatively low frequency square wave current in order to achieve an efficacy which is equivalent to that obtained with sine-wave operation at higher frequencies. Such a mode of operation for fluorescent lamps is disclosed in "Illuminating Engineering", Vol. 54, January 1959, p. 69, discussion of article by Campbell.